Extract (headers, exchange, messages) relay loops into separate crates (#357)

* extracted relay crates

* moved metrics to utils

* exchange-relay compilation

* fix compilation of headers-relay

* fixed messages-relay compilation

* fixed ethereum-poa-relay compilation

* cargo lock

* cargo fmt --all

* clippy

* cargo fmt --all

* fix tests compilation

* clippy

* eof

* module level docs

* removed obsolete comment

* #![warn(missing_docs)]

* .0 -> Deref

* post-merge fix

* cargo fmt

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

* Update relays/headers-relay/src/lib.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

bridges/relays/ethereum/Cargo.toml

@@ -8,9 +8,7 @@ license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
 [dependencies]
 ansi_term = "0.12"
 async-std = "1.6.2"
-async-stream = "0.3.0"
 async-trait = "0.1.40"
-backoff = "0.2"
 bp-currency-exchange = { path = "../../primitives/currency-exchange" }
 bp-eth-poa = { path = "../../primitives/ethereum-poa" }
 clap = { version = "2.33.3", features = ["yaml"] }
@@ -20,18 +18,18 @@ ethabi = "12.0"
 ethabi-contract = "11.0"
 ethabi-derive = "12.0"
 ethereum-tx-sign = "3.0"
+exchange-relay = { path = "../exchange-relay" }
 futures = "0.3.5"
+headers-relay = { path = "../headers-relay" }
 hex = "0.4"
 hex-literal = "0.3"
-linked-hash-map = "0.5.3"
 log = "0.4.11"
+messages-relay = { path = "../messages-relay" }
 num-traits = "0.2"
 parity-crypto = { version = "0.6", features = ["publickey"] }
-parking_lot = "0.11.0"
-rustc-hex = "2.0.1"
+relay-utils = { path = "../utils" }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0.57"
-sysinfo = "0.15"
 time = "0.2"
 web3 = "0.13"